Cloth Deforms When I Move Character
I'm not a cloth expert but as I told you I think this tutorial by Aneesh Arts gives a series of good tricks.
The first thing I would say is that your gown and pants are rather high-poly and it doesn't help to easily work, you should do a low-poly version and give it a Subdivision Surface modifier.
That said I was able to quickly improve your problem with these 2 solutions:

First, move a bit the vertices of the bottom of the gown with the Proportional Editing enabled. You can even create a new shapekey so that the topology only moves when necessary:



Find and display the deforming bones in the armature layers:



Select the armature, then shift shelect the pants, switch to Weight Paint mode, check each bone and use Blend &gt; Mix or Subtract to increase or decrease their influence. Then do the same for the gown, until there is no clothes overlapping:



At the end you should be able to have a good result:


There is another method that may help, it is to give your clothes a Mesh Deform modifier as I explained here, you could try to associate it with cloth simulation...